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Ex2. Fold And Fold Again

Related Topics: Recursion, Function pointer.

In this exercise, you will need to implement the fold function below, where count is the number of elements in the arr array, fn points to a function, which takes in two ints and returns an int, and initial is a value. You are welcomed to write a helper function for the recursion.
```cpp
int fold (int count, int arr[], int (*fn) (int, int), int initial) {

}
```

If count == 0, just return the initial value. Otherwise, take count = 3 for example, the return value could have been equivalently calculated as follows:

int temp1 = fn(arr[0],initial)
int temp2 = fn(arr[1],temp1)
int temp3 = fn(arr[2],temp2)
return temp3

In short, the fold function will take each element in the array as well as the previous return value, until we have gone through the whole array. And it will return the last calculated value.

To help you understand how the fold function works, write two fn functions to be used with the fold function:
- a function called fn_add such that fold(n, arr, fn_add, 0) returns the sum of the elements in arr.
- a function called fn_count_odd such that fold(n, arr, fn_count_odd, 0) returns the number of odd numbers in arr.

Hint: Please be aware that the order of inputs into the fn function matters. The first one should be the element from the array, and the second one should be either the initial value or the previous return value of fn.

Requirement: Please refer to ex2.cpp in the starter_files folder, you will need to implement fold, fn_add and fn_count_odd functions. You only need to submit ex2.cpp for this problem.